Transcript Chapter 12 Chemical Bonding - California Lutheran University

Chemical Bonding
I. What is a Bond?
A force that holds atoms
together in a molecule or a
compound
Hydrogen atoms bonded together
II. Why are bonds important?
A. Almost everything in the
world around us is held
together by chemical
bonds.
B. The way in which atoms are
bound together has a great
effect on chemical and
physical properties of
substances.
1. Example: graphite and
diamond are both composed
solely of carbon yet they are
completely different in their
properties due to the different
types of bonds between the
carbon atoms.
Bonding is planar in graphite
Bonding is tetrahedral in diamond
C. Molecular bonding and
structure play a key role in
determining the course of all
chemical reactions, many of
which are vital to our
survival.
Example: The bonds in DNA
determine its shape and how it is
able to transmit genetic
information into protein.
DNA side view
DNA end view
Example: The bonds in various
enzymes determine what molecule
they will react with. The amylase
enzyme will react only with starch ,
sucrase only with sucrose and ATP
synthetase only with ADP and P.
Sucrose
Sucrose being split
Sucrose
Alpha amylase
III. Why do bonds form?
A major reason is to give
more stable arrangements of
electrons within atoms
involved in the bond.
IV. What is involved in forming
bonds?
A. Electron configuration this determines whether
an atom will gain, lose or
share electrons in
forming a bond.
B. Electronegativity
1. Definition - the relative
tendency of an atom to attract
electrons to itself when it is
bonded to another atom.
2. Four factors affecting it
include: (1) nuclear charge
(2) shielding effect (3)
Radius and (4) sublevel
3. Trends in electronegativity:
Follows the same trends
observed in ionization energy
and electron affinity
(Decreases down a group and
increases left to right across the
periodic table). Electronegativity
is a unitless value since it is a
ratio of the same property for two
elements.
C. Electronegativity and Bond
Character.
1. The greater the
electronegativity difference
between the two bonding atoms
the stronger the bond is that
forms between them.
2. If the difference is greater
than 1.67 an ionic bond
forms. If it is less than 1.67 a
covalent bond forms. Most
bonds are a mixture of both types.
V. Types of Bonds and the
materials they form.
A. Ionic Bonds
1. Characterized by a transfer
of electrons. When electrons
are transferred between atoms
ions are produced having
opposite charges. The
attraction of oppositely
charged ions holds them
together. This electrostatic
attraction is the ionic bond.
2. Characteristics of ionic
compounds include high melting
points, ability to conduct
electricity in the liquid and
gaseous states, soluble in water,
and they usually crystallize as
sharply defined particles.
3. They are typically composed
of a metal and a nonmetal.
4. An example of a substance
having ionic bonds is NaCl,
sodium chloride. (Watch the bond
being formed in the
demonstration below)
Na Na Cl
Cl
B. Covalent Bonds
1. Characterized by a sharing
of electrons. The shared pair
of electrons make up the
covalent bond.
2. Most covalent compounds
are formed between atoms of
nonmetals
3. Examples are H2, H2O,
NH3 and CH4
H H
Single bond
Triple bond
Double bond
Quadruple bond
4. Characteristics of Covalent
Compounds include (1) low melting
points (2) do not conduct electricity
in any state (3) solids are brittle.
5. The particle that results from a
covalent bond is a molecule.
6. Definitions relating to molecules:
a. bond axis - the line joining the
nuclei of two bonded atoms
b. bond angle - the angle between
the bond axes when three or more
atoms are bonded together.
c. bond length - the distance
between nuclei along the bond axis.
Bond Length
Bond Angle
Bond Axis
7. Bond length is not fixed
because the atoms have a variety
of movements such as bending,
wagging, stretching and
rotational vibrations. These
movements cause bond angles
and bond lengths to vary. What
we actually measure is an
average.
C. The Metallic Bond
1. Characteristics of metals include
high electrical conductivity, luster or
shininess, and malleability.
2. Electrons in the metallic bond
are said to be delocalized because of an
overlap of outer level orbitals between
atoms. This allows electrons the
freedom to move from atom to atom.
These delocalized electrons make up
the metallic bond.
Sodium atoms in metallic sodium.
3. The properties of metals are
determined by the number of
electrons they can contribute to the
metallic bond.
4. Group IA metals have only one
electron to contribute to the bond and
are therefore soft. Group IIA with
two electrons are a little harder.
5. The hardest metals are found in
groups VIB through VIIIB where
electrons from partially filled “d”
sublevels take part in the metallic
bond.
Examples of period 4 metals follow in
order of increasing atomic number
C h em i c a l B o n d
B o nd
F o rm e d
F o rm e d B y
T y pe
B e tw e e n
C o v a l e n tA t o m s o f
S h ar i n g of
n o n- m e t a lle
il
cec t r o n
e l em e n t s op
fa ir s
s i mi l a r
e l ec t r o nt
eg a
i v it y
S u m m ar y
P r o p e r t i e sE x a m p l e s
of
S t a b l e n o nH-C l , 2OH ,
i o ni z i n g
N H3 , C 4H
m o le c u l e snot
c o nd u c t o rs
of
e l ec t r i c it y
in any
p h as e . L ow
m e lt i n g
p o in t
I o ni c
A t om s o f
E l e c t r o s t a t Ci hc a r g e d
N a Cl ,
m e ta l l i c aa
nt
d t r a c t i o n i o n s . S o l iBda S , L i Hd, C
n o nm e t a l lib
c e t w e e n i o n ds o e s n ’ t
O , Na F
e l em e n t s or
fe su l t i n g
c o nd u c t
w i de l y
f r o m t r a n s f ee lr e c t r i c i t y
d i f f e r e n t o f e l e c t r o n ls i q u i d a n d
e l ec t r o nt
eg a
gas do.
i v it i e s
H i gh
m e lt i n g
p o in t s
M e t a l l i cA t o m s o f
D e l o c a l i z e d A l l p h a s e sN a , K , C u ,
m e ta l l i c
e l ec t r o n
are
Z n , V , M n,
e l em e n t s
c l o u d a r o u n ed l e c t r i c a lF e , C o , N i
a t o m s o f l o cw o n d u c t o r s ,
e l e c t r o n eig a lt u s t r o u s ,
v i ty
v e ry h i g h
m e lt i n g
p o in t s